(1) Field of the Invention
The present invention relates to a non-aqueous electrolyte cell comprising a positive electrode, a negative electrode, and an electrolyte consisting of a solute and an organic solvent, all contained in a cell can, in which lithium trifluoromethanesulfonate is employed as the solute.
(2) Description of the Prior Art
A non-aqueous electrolyte cell in which the negative electrode has lithium, sodium or an alloy thereof as an active material provides the advantages of high energy density and low self-discharge rate. However, this type of cell is inferior in low temperature discharge characteristics and has room for improvement in this respect.
In view of the above situation, proposals have been made to improve the low temperature discharge characteristics of the lithium cell by using lithium trifluoromethanesulfonate (LiCF.sub.3 SO.sub.3) as the solute, which is highly soluble in a non-aqueous solvent and does not cause deposition of lithium salt on the negative electrode during low temperature discharge.
Where lithium trifluoromethanesulfonate is used as the solute, initial low temperature discharge characteristics are improved but there is a problem of deterioration in low temperature discharge characteristics after a long storage period. The deterioration takes place for the following reasons:
(1) When this type of cell is stored for a long period, a reaction occurs between fluorine ionized from lithium trifluoromethanesulfonate and the lithium which is the active material of the negative electrode. As a result, a layer of lithium fluoride which is a passive substance is formed on the negative electrode surface, thereby increasing internal resistance of the electrode.
(2) A cell can becomes corroded during the storage period due to fluorine ionized from lithium trifluoromethanesulfonate, and metallic ions formed by the corrosion deposit on the negative electrode surface, thereby increasing internal resistance of the electrode.
Meanwhile, the following cells have been proposed:
(1) Cells in which the electrolyte includes a solvent mixture of propylene carbonate and 1,2-dimethoxyethane (U.S. Pat. Nos. 4,279,972 and 4,482,613); a cell in which the electrolyte includes a solvent mixture of propylene carbonate, 1,2-dimethoxyethane and 1,3-dioxolane (U.S. Pat. No. 4,129,691); a cell in which the electrolyte includes a solvent consisting of dimethylformamide (U.S. Pat. No. 4,142,028); a cell in which the electrolyte includes a solvent mixture of propylene carbonate and tetrahydrofuran (Japanese Patent Publication Kokai No. 60-243972); and a cell in which the electrolyte includes a solute consisting of lithium perchlorate and a solvent mixture of propylene carbonate and 1,2-dimethoxyethane (Japanese Patent Publication Kokai No. 60-86771).
(2) A cell having a can formed of stainless steel, and in particular ferritic stainless steel containing almost no nickel.
However, the cells listed in paragraph (1) above do not provide sufficient improvement in the low temperature discharge characteristics after storage yet.
The cell in paragraph (2) fails to solve the problem of metal corrosion to the full extent.
Thus the cells proposed heretofore do not provide sufficient improvement in the low temperature discharge characteristics after storage.